Who Decides What Time It Is Where You Are
When your phone moves its clock an hour because a government changed the rules, something had to tell it. That something is not a treaty organisation or a standards body in the usual sense. It is a database maintained largely by volunteers, and the story of how it works is stranger than the subject deserves.
The database behind every clock you own
The IANA Time Zone Database, also called tzdb, zoneinfo or the Olson database after its founder Arthur David Olson, is the authoritative record of which places use which offsets, when they change, and what they did historically. It has been coordinated for many years by Paul Eggert.
Essentially everything uses it. Linux, macOS, iOS, Android and Windows all derive their
time zone handling from it, directly or indirectly. So do Java, Python, Ruby, PHP and the
Intl APIs built into every modern browser, which is what this site uses to
compute times in your own device rather than on a server.
It is, by any reasonable measure, one of the most widely depended-upon pieces of public infrastructure in computing, and it runs on a mailing list.
What a "time zone" means here, precisely
The database's definition is narrower and stranger than the everyday one. A zone is a region whose clocks have agreed on everything, including daylight saving rules and historical changes, since 1970.
The 1970 cutoff is not arbitrary: it is the Unix epoch, the moment computers conventionally count time from. Agreement before then does not matter for the database's purposes.
This explains something that confuses most people looking at the list for the first time. The United States appears to have four or five time zones. The database has far more American identifiers than that, because places that show identical times today may have diverged at some point after 1970. America/New_York and America/Detroit read the same clock now, but Detroit's history differs, so they are separate entries. The same logic is why Arizona needs its own identifier: it stopped observing daylight saving while the rest of the Mountain zone kept it.
Why zones are named after cities
Identifiers take the form Area/Location: Europe/London, America/New_York, Asia/Kolkata, Australia/Adelaide. The convention looks fussy and is deliberate.
Countries were rejected as the naming unit for two reasons. Many countries contain several zones, so the name would not be unique. And country names and borders change more often than people assume, which would mean renaming identifiers that software depends on.
Cities are more stable, and the chosen city is usually the largest in the zone rather than the political capital, so the name is recognisable. That is why India's zone is Asia/Kolkata rather than Asia/New_Delhi, and why the US eastern zone is America/New_York rather than something referring to Washington.
Cities do occasionally rename, and the database handles it by adding the new name and keeping the old one as a permanent alias. Asia/Calcutta still resolves correctly, decades after Asia/Kolkata became the primary name, because breaking existing systems is considered worse than carrying the history.
How little warning governments give
Releases are versioned by year and letter, such as 2024a and 2024b, and they come out whenever reality demands rather than on a schedule. Some years see a handful, some many more.
The reason is that governments change their clocks with remarkably little notice. Egypt has reinstated daylight saving with roughly a week's warning. Morocco adjusts its clocks around Ramadan each year, on a date that shifts annually. Countries have announced changes taking effect in under a fortnight.
Each time, the database has to be updated, the release has to reach operating system vendors, the vendors have to ship an update, and the update has to reach your device. That chain takes time, and it is why a phone that has not been updated in a while can display a confidently wrong local time for a country that changed its rules last month.
It is also the honest limit of a site like this one. Every time shown here is computed in your browser from your device's own copy of that data, which means it stays correct through daylight saving transitions with nothing to update on our side. It also means that if your device's data is stale, the figure shown is stale with it. For anything where being wrong carries a real cost, confirm against an official source for that country.
The lawsuit that nearly deleted it
In 2011 the database was hosted at a US government research institution and maintained, essentially, as a volunteer effort. That September, a company that sells astrology software sued Olson and Eggert, claiming that historical time zone data in the database infringed the copyright of an atlas it had rights to.
The immediate effect was that the mailing list and the server were taken offline. For a short period, the reference copy of the data underpinning timekeeping across the software industry was simply not available.
The Electronic Frontier Foundation took up the defence, arguing that factual information about what time a jurisdiction kept cannot be copyrighted. The suit was withdrawn in early 2012.
The lasting consequence was institutional. The database moved to IANA, the body that also administers domain name and IP address registries, giving it a stable home and some legal backing. That is why it is now called the IANA Time Zone Database rather than the Olson database, and the episode is the reason anyone thought to give it an institution at all.
What this means in practice
Three things follow from all of this, and they are worth internalising if you work across time zones.
- Time zone rules are policy, not physics. They change, sometimes at short notice, and no stored calculation about a future local time is permanently safe.
- Identifiers are the stable thing, offsets are not. Europe/London is a durable fact. UTC+0 is a fact about London only part of the year.
- Keep your devices updated. Time zone corrections arrive through ordinary operating system updates, and skipping them is how you end up an hour out in a country that changed its mind.
The country index on this site lists which zones each country's cities use, with every figure computed from that same database through your browser. The about page explains the data sources in more detail.